Reduction of the 9-keto group of the glutathione conjugate of PGA1 was catalyzed by red blood cell suspensions, lyzed red cells, and purified 9-hydroxyprostaglandin dehydrogenase. The latter reaction is 24,000 times faster than the corresponding reduction of PGE1. PGA1 infused into the canine renal artery appeared in urine in the form of polar conjugates with the chromatographic properties of cysteine and N-acetyl cysteine PGA1. PGE1 and PGF1 alpha were converted to less polar metabolites in similar experiments. Evidence was obtained for the occurrence in human urine of a conjugate of PGA2.